The present invention relates to a method for reducing material containing metal oxide in solid phase in a circulating fluidized bed reactor.
The present invention is particularly suited for reduction of iron ore to metallic iron with carbon, i.e. with a mixture of CO and CO.sub.2. The invention can advantageously be used for pre-reducing iron ore before the smelting stage in a direct smelting reduction process.
The reduction of iron oxide is an endothermic process and requires supply of energy. In a reduction process in which coal or coke in solid form is supplied, the energy required for the reaction can easily be supplied by partial combustion of the coal. Depending on the temperature, a certain content of CO.sub.2 in the gas can be permitted, preferably however so that the CO.sub.2 /CO+CO.sub.2 ratio does not exceed 0.2. This implies a certain degree of oxidation of the coal or the coke beyond the CO stage, but requires then preheating of the ore concentrate as well as the air, if air and not oxygen is used.
The reaction kinetics of the reduction EQU Fe.sub.2 O.sub.3 .fwdarw.FeO
is relatively unfavourable at the low temperatures normally prevaling in fluidized bed reactors. At temperatures of about 800.degree. C., reaction times of several minutes, possibly tens of minutes, are required, depending on the particle size and the desired degree of reduction. The subsequent reaction according to EQU FeO+CO.fwdarw.Fe+CO.sub.2
to metallic iron is effected at a temperature of above 700.degree. C. at an appropriate gas composition.
The reduction of iron ore to metallic iron in the fluidized bed is impeded by the tendency of the particles in the bed to sinter. Higher temperatures, which would give higher, and therefore more favourable, reaction kinetics for the reduction process, lead to a higher tendency to sinter. The risk of sintering has considerably limited the use of fluidized bed technique for pre-reduction of iron ore.
Sintering is believed to be caused in part by the sticky iron ore particles in which the iron is completely or partly in metallic form. FeO appears as a molten layer on the surface of the pre-reduced ore, which causes sintering of small particles into larger particles and aggregates. Sintering of the particles in the reactor renders it diffucult or impossible to bring about fluidization in the reactor.
Sintering can, in addition to a molten iron layer on the particles, be caused by crystallization of metallic iron as dendrites on the ore particles, whereby particles are formed that very easily become attached to and grow into each other. Sintering is also believed to be caused by a particularly active layer of metallic iron surrounding the larger ore particles, the active layer having a certain adhesion force and attracting smaller particles.
Sintering can be avoided by carrying out the reduction at very low temperatures, which however would result in unfavourable reaction kinetics and, at lower temperatures, in formation of carbides instead of metallic iron.
To avoid sintering in reduction in a fluidised bed at higher temperatures, coal or coke has been mixed in, which has been believed to prevent sintering, either in form of indvidual particles in the bed or in form of a protecting coke layer on the bed particles. Injection of oil in the hot bed has also been believed to contribute to the formation of a layer of coke on the iron particles, which would prevent sintering.
Addition of coke has, however, proved to cause segregation, particularly in conventional fluidized beds, so that the iron particles concentrate in the lower part of the reactor and the coke particles in the upper part of the reactor. This has had a negative effect on the reduction process.
It is an object of the present invention to provide a method for reducing material containing metal oxide in which the above mentioned drawbacks, i.e. segregation and sintering, can be avoided.
The present invention has in a surprisingly simple manner solved the problems of the reduction processes described earlier by carrying out the reduction in an circulating fluidized bed (CFB) reactor so that
coal or coke in excess, for reduction of the material containing metal oxide, and gas containing oxygen gas is introduced in the fluidization chamber of the reactor so as to bring about generation of heat for maintaining a temperature of &gt;850.degree. C. in the fluidization chamber; PA1 bed material containing pre-reduced material containing metal oxide and coke is exhausted with the flue gases through a gas outlet in the upper part of the fluidization chamber and conveyed to a particle separator and cooled to a temperature equal to or &lt;850.degree. C.; PA1 the bed material which has been separated from the flue gases in the particle separator is returned to the lower part of the fluidization chamber via a carbidization chamber in which conditions favourable for formation of carbide are maintained. PA1 high reaction kinetics for the reduction, while the reduction process in a CFB reactor can be effected at relatively high temperatures, and PA1 formation of carbide which prevents sintering brought about by an decrease of the temperature in the recirculation step, by direct cooling before, after or in the particle separator or brought about by the endothermic reduction reactions.
According to the method of the invention, by supplying coal or coke in excess and a certain amount of gas containing oxygen gas to a CFB reactor, heat can be generated and a high temperature be maintained in the fluidization chamber. The gas containing oxygen gas can consist of air preheated to a temperature of &gt;800.degree. C., preferably &gt;1000.degree. C., oxygen-enriched air or pure oxygen gas. This results in high level reaction kinetics, whereby, with an appropriate CO.sub.2 /CO+CO.sub.2 ratio, metallic iron is produced according to the reaction EQU FeO+CO.fwdarw.Fe+CO.sub.2.
Lowering the CO.sub.2 /CO+CO.sub.2 ratio results in reduction of iron oxide on the surface of the particles of the ore concentrate according to the carbidization reaction EQU FeO+4C.fwdarw.Fe.sub.3 C+3CO
which is favorable as regards the sintering. The formation of iron carbides takes precedence of the formation of metallic iron. This is also promoted by lower temperatures.
According to the invention, the above mentioned carbidization reaction is used in the recirculation system of the CFB reactor. In the return pipe and the carbidization chamber pre-reduced iron ore and coke which has been separated from the flue gases of the reactor will be in an unfluidized state, the gas atmosphere which surrounds the particles consisting mainly of pure CO, the CO.sub.2 /CO+CO.sub.2 ratio consequently being very small. The CO atmosphere which surrounds the particles is obtained by the reduction reactions which continue in the recycled material in the recirculation system. As the temperature of the material at the same time decreases some tens of degrees (possibly 100 degrees), either by cooling or only because the endothermic but not the exothermic reactions continue, the reduction products of in the recirculation system of the CFB reactor will consist of Fe.sub.3 C in accordance with the reaction formula above. A temperature of 800.degree. to 850.degree. C. is in most cases suitable. The dwell time in the reactor can be influenced by modifying the design of the return pipe.
A formation of carbide on the surface of the partly reduced ore concentrate will prevent sintering of the material in the recirculation part as well as in the fluidization part of the CFB reactor. The invention renders it possible to prevent sintering of the particles in the bed without causing detrimental effects on the reaction kinetics of the reduction process in the fluidization chamber.
By means of the method of the present invention, the undesired sintering in a fluidized bed reactor can be brought under control, irrespective of the form of the metallic iron produced by the reduction, be it pure Fe or Fe.sub.3 C. If this process is used as a primary stage in a direct smelting process, possible carbides in the reduced material will have a positive effect on the whole process.
The invention brings about inter alia the following advantages:
Pre-reduction of iron oxide requires a certain minimum of reduction potential of the reducing gas. For instance in a reduction process according to the invention in a reactor with a circulating fluidized bed having a particle size of up to 1 mm and a temperature of 900.degree. C., a CO.sub.2 /CO+CO.sub.2 ratio of between 0.2 and 0.3 can give a reaction time of some minutes, e.g. 10 minutes, and an acceptable degree of metallization of iron ore.
The invention will be further described with reference to the accompanying drawing showing an apparatus for carrying out the method according to the invention.